Recovery of hydrocarbon materials from earth formations by application of heat



May 29, 1962 R. L. KOCH ETAL 3,035,632

RECOVERY OF HYDROCARBON MATERIALS FROM EARTH FORMATIONS BY APPLICATIONOF HEAT Filed Dec 24, 1958 2 SheetsSheet l FIG. I.

FIG.2.

ROBERT L. KOCH HENRY J. RAMEY RODMAN JENKINS INVENTOR.

gLd

ATTORNEY May 29, 1962 R. 1.. KOCH ET'AL 3,036,632

RECOVERY OF HYDROCARBON MATERIALS FROM EARTH FORMATIONS BY APPLICATIONOF HEAT Filed Dec. 24, 1958 2 Sheets-Sheet 2 FIG. 4.

ROBERT L. KOCH HENRY J. RAMEY RODMAN JENKINS INVENTOR.

ATTORNEY United States Patent 50 3,036,632 RECOVERY OF HYDROCARBONMATERIALS FROM EARTH FORMATIONS BY APPLICA- TION OF HEAT Robert L. Koch,Dailas, Tex., Henry J. Ramey, .lr., Fullerton, Calif, and RodmanJenkins, Mount Kisco, N.Y., assignors, by mesne assignments, to SoconyMobil Oil goniipany, Inc., New York, N.Y., a corporation of New FiledDec. 24, 1958, Ser. No. 782,819 5 Claims. (Cl. 166-11) This inventionrelates to the recovery of hydrocarbon materials from earth formationsand relates more particu' larly to the recovery of hydrocarbon materialsfrom earth formations by the application of heat thereto, said heatbeing obtained by combustion in place of a portion of the hydrocarbonmaterials in the earth formation.

Petroleum oil is usually recovered initially from an earth formationcontaining such hydrocarbon material as a result of gas pressure ornatural water drive forcing the oil from the oil-bearing formationthrough a producing well leading therefrom to the surface of the ground.As recovery of oil from the formation continues, the energy within theformation gradually decreases and finally becomes insufficient to forcethe oil to the surface of the ground, although a large portion of theoriginal quantity of the oil remains in the formation. To increase theulti* mate recovery of the oil, pumping is then employed, but when therate of recovery by pumping falls to an uneconomically low level, afurther increase in the ultimate recovery of the oil may still beeconomically effected by the employment of such recovery methods as gasdrive or water drive. It has recently been proposed to increase theultimate recovery of the oil by a combustion process.

In the combustion process, an oxidizing gas is passed, as by pumping,through an input Well or input Wells to the formation and combustion ofthe oil within the formation is initiated by suitable means. A zone ofcombustion advances from the input well or input wells to an output wellor output wells leading from the formadon to the surface of the ground.As the combustion zone advances with continued supply of oxidizing gasto the formation through the input Well or wells, combustion gases, oil,and distillation and viscosity breaking products of the oil advance infront of the combustion zone to the output Well or wells from which thefluids are removed and thereafter treated for recovery of the desiredvaluable constituents. The heat of the fluids advancing in front of thecombustion zone strips the oil-bearing formation of the greater portionof the oil, leaving behind within the formation a carbonaceoushydrocarbon deposit and the carbonaceous deposit essentially is the fuelconsumed in the process.

While the combustion process is effective for the recovery of oil fromearth formations, it suffers to some extent from various drawbacks. Forexample, it is necessary to supply continually oxidizing gas to theinput Well or wells in order to maintain combustion and the provision ofsufficient compressor capacity to supply suflicient oxidizing gas plusthe costs of operation of the compressors represent a not inconsiderableitem of expense. Further, control of operating variables is requiredthroughout the entire process in order to obtain the maximum yield ofoil commensurate with economy of operation which repre sents anotherappreciable item of expense.

It is an object of this invention to provide an improved method ofrecovering hydrocarbon materials from earth formations by the combustionprocess. It is another object of this invention to recover hydrocarbonmaterials from earth formations by the application thereto of heat. Itis another object of this invention to reduce the cost of recoveringhydrocarbon materials from earth formations where recovery is obtainedby application of heat. It is another object of this invention to reduceoperational control of process variables in the combustion process forthe recovery of hydrocarbon materials from earth formations. Otherobjects of the invention will become apparent from the followingdetailed description thereof.

In accordance with the invention, combustion is initiated in an earthformation containing hydrocarbon material, oxidizing gas is passed intothe formation through an input well leading thereto whereby thecombustion zone is advanced through the earth formation from the inputwell to an output well leading therefrom, thereafter the supply ofoxidizing gas to the earth formation in the direction of the output wellis discontinued when the combustion zone has advanced through theformation less than the entire distance from the input well to an outputwell, the heat in the portion of the formation already traversed by thecombustion zone is allowed to transfer therefrom, without any assistancefrom injection of fluid, into the portion of the formation in thedirection of an output well not yet traversed by the combustion zone,and hydrocarbon material is recovered from the output well.

FIGURE 1 is a diagrammatic representation of an earth formationpenetrated by an input well and four output wells symmetrically arrangedabout the input well.

FIGURE 2 is a diagrammatic representation of a portion of the earthformation of FIGURE 1 indicating tem perature distribution at the timethe supply of oxidizing gas to the formation is discontinued.

FIGURE 3 is a diagrammatic representation of the same portion of theformation as shown in FIGURE 2 indicating temperature distribution at atime after the supply of oxidizing gas is discontinued and heat transferhas occurred without assistance from injection of fluid.

FIGURE 4 is a diagrammatic representation of the same portion of theformation as in FIGURE 1 indicating the temperature distribution at astill later time after the supply of oxidizing gas is discontinued andheat transfer has occurred without assistance from injection of fluid.

We have found that, by procedure of the invention, the hydrocarbonmaterial in the portion of the formation in the direction of the outputwell not yet traversed by the combustion zone is raised in temperature,Without the necessity of injection of fluid, to a point where improvedoil recovery is obtained by the resulting lowering of the viscosity ofthe hydrocarbon material. For example, with a hydocarbon material suchas 14 API crude oil, a viscosity change from about 5000 -to about 200centipoises will be effected by increasing the temperature of the oilfrom about 60 F. to F. A viscosity change of this order of magnitudepermits a much easier flow of the oil through the formation withconsequent increased ultimate yield of oil.

Herein-above as well as hereinafter, advance of the combustion zone fromthe input well to the output well shall mean the maximum distancetraversed by the combustion zone along a straight line extending fromthe output well to the input well.

In carrying out the invention oxidizing gas is passed into the formationthrough the input well in the direction of the output well until thecombustion zone has advanced, by virtue of the formation being suppliedWith oxidizing gas, a distance equal to at least forty percent of thedistance from the input well to the output well from which hydrocarbonmaterial is to be recovered. The oxidizing gas may be passed into theformation in the direction of the output well until the combustion zonehas advanced, by virtue of the formation being supplied with oxidizinggas, more than forty percent of the distance from the input well to theoutput well but ordinarily it is preferred that oxidizing gas be passedinto the forma tion in the direction of the output well no longer thanis required to advance the combustion zone ninety percent of thedistance from the input well to the output well. In the practice of theinvention, the oxidizing gas is passed continuously into the formationin the direction of the output well from the time ignition of thehydrocarbon material in the formation is first effected until thecornbustion zone has advanced the desired distance. Thereafter, heat ispermitted to transfer by conduction Without any assistance frominjection of fluid in the direction of the output well from the portionof the formation traversed by the combustion zone to the portion of theformation not yet traversed by the combustion zone. However, if desired,the oxidizing gas may be passed intermittently into the formation in thedirection of the output well until the total of the increments of thedistances advanced by the combustion zone while oxidizing gas is be ingpassed into the formation is equal to the desired distance between theinput well and the output well.

In the combustion process, the combustion zone advances from the inputwell to the output well during the time that oxidizing gas is beingpassed to the formation in the direction of the output well. Byconduction during the time that gas is not being passed to the formationin the direction of the output well, the portion of the formation lyingin front of the combustion zone may be heated above the ignitiontemperature of the hydrocarbon material contained therein. However, thisis not to be regarded as advance of the combustion zone, and, whereintermittent operation is employed, the total time that oxidizing gas ispassed into the formation in the direction of the output well is suchthat the accumulated distance of advance of the combustion zone witheach passage of oxidizing gas in the direction of the output Well isequal to the desired distance.

An essential step of the invention is discontinuance of the supply ofoxidizing gas to the earth formation in the direction of the output welland allowance of transfer of the heat without any assistance frominjection of fluid from the portion of the formation already traversedby the combustion zone to the portion of the formation in the directionof an output well not yet traversed by the combust-ion zone. This stepinvolves not only shutting off the supply of oxidizing gas but stoppingflow of any liquid or gaseous medium into the input well through theformation in the direction of the output well. As a result of this step,the hydrocarbon material in the portion of the formation in thedirection of the output well not yet traversed by the combustion zone isincreased in temperature with resulting ease of flow and recovery, ashereinabove mentioned. But, this increase in temperature is obtainedwithout the necessity of any expenditure of energy as would be involvedin maintaining combustion during this time by passing oxidizing gas tothe formation in the direction of the output well or by transferringheat during this time from the portion of the formation traversed by thecombustion zone to the portion of the formation in the direction of theoutput well not yet traversed by the combustion zone by passing an inertfluid through the formation in the direction of the output well.Further, no control of process variables is required while heat is beingtransferred without assistance from injection of fluid.

Transfer of heat from the portion of the formation already traversed bythe combustion zone to the portion of the formation in the direction ofthe output well not yet traversed by the combustion zone without anyassistance from injection of fluid may cause production of hydrocarbonmaterial from the output well depending upon the properties of thehydrocarbon material within the formation, the temperatures attainedwithin the formation, and the proximity of the combustion zone to theoutput well. However, production of hydrocarbon material from the outputwell may not occur as a result of this transfer of heat. It is,therefore, necessary, follow 4 ing the transfer of heat, to pass a fluidfrom the input well through the formation to the output well to drivethe hydrocarbon material to the output well in order to obtainproduction therefrom.

In the practice of the invention, transfer of heat Without anyassistance from injection of fluid from the formation already traversedby the combustion zone to the formation in the direction of the outputWell not yet traversed by the combustion zone is permitted to occur fora period of time. This period of time may vary but must be appreciablein orde that the benefits of permitting heat to transfer without anyassistance from injection of fluid are obtained. Accordingly, thisperiod must be at least as great as one month, or thirty days. In apreferred embodiment of the invention, this period, which is at least asgreat as one month, is also sufliciently great that heat will havetransferred through the formation Without assistance from injection offluid to the extent that the temperature of the formation at the outputwell will have risen measurably above its temperature existing at thetime the supply of oxidizing gas, and combustion, Was discontinued. By ameasurable rise of temperature is meant an increase in temperature whichis detectable by instruments ordinarily employed in the art ofrecovering hydrocarbon materials from earth formations by theapplication of heat. These instruments include thermometers orthermocouples. A preferred rise in temperature is about 50 F.

Following the step of transfer of heat without any assistance frominjection of fluid from the formation already traversed by thecombustion zone to the formation in the direction of the output well notyet traversed by the combustion zone, a fluid is injected into the inputwell through the formation to the output well to drive hydrocarbonmaterial within the formation to the output well. By the term fluid,gases as well as liquids are included. This step of injecting fluid intothe input well through the formation to the output well, in order thatthe benefits of the invention with respect to control of processvariables are obtained, must be carried out under conditions thatcombustion will not occur. Thus, this step may be carried out employinga fluid which will not support combustion. Such fluids include carbondioxide, water, hydrocarbons such as methane, etc.

This step of injecting fluid into the output well through the input wellthrough the formation to the output well may also be carried outemploying a fluid, such as air, which will support combustion. However,where a fluid which will support combustion is employed, in order thatthis step be carried out under conditions that combustion will notoccur, the maximum temperature within the formation in the path of theflow of fluid, and the temperature of the fluid upon entrance into theformation, must be below the ignition temperature of the hydrocarbondeposit. Accordingly, where, following the step of transfer of heatwithout any assistance from injection of fluid, a fluid which willsupport combustion is employed to drive hydrocarbon material within theformation to the output well, the period of time during which transferof heat without any assistance from injection of fluid is permitted tooccur must be sufliciently great that the maximum temperature betweenthe input well and the output well in the path of the flow of fluid isbelow the ignition tempera ture of the hydrocarbon deposit within theformation. Ignition temperatures of the hydrocarbon deposit within theformation may vary depending upon the character of the deposit. However,ordinarily, the ignition temperature of the hydrocarbon deposit will beat least as great as 400 F. Thus, where a fluid which will supportcombustion is to be employed to drive hydrocarbon material within theformation to the output well, the period of time that heat is permittedto transfer without assistance from injection of fluid must not only beat least as great as one month but also must be suihciently great thatthe 5, maximum temperature within the formation is not in excess of 400F.

A formation being treated by the subsurface combustion process may haveone input well leading thereto and one output well leading therefrom. Inmost instances, however, the formation will have a plurality of outputwells leading therefrom and the output wells are ordinarily arranged ina symmetrical pattern around the input well. In carrying out the processof the invention where the earth formation has only one output wellleading therefrom, the supply of oxidizing gas to the formation isdiscontinued when the combustion zone has advanced the desired distancebetween the input well and the output well. On the other hand, incarrying out the process of the invention where the earth formation hasa plurality of output wells leading therefrom, the supply of oxidizinggas is discontinued by imposing a back pressure successively upon eachof the output Wells when the combustion zone has advanced the desireddistance from the input well to each individual output well whereby theadvance of the combustion zone to that output well is halted. Theimposition of the back pressure is most conveniently effected by closingoff each well although other methods may be employed if desired. Whenback pressure has been imposed upon the last of the output wells leadingfrom the formation, the supply of oxidizing gas is discontinued. In theevent the combustion zone advances at a uniform rate between the inputwell and each of the output wells, the supply of oxidizing gas isdiscontinued when the combustion zone has traversed the desireddistance.

The following will be illustrative of the invention.

Referring to FIGURE 1, a portion of an earth formation delineated byfour output wells leading therefrom is indicated generally by thenumeral 10. Output wells 11, 12, 13, and 14 lead from the formation andare ar ranged in a symmetrical pattern, namely, a square, about inputwell 15 leading to the formation. The temperature of this formation is100 F. Combustion is initiated in the formation at the input well 15 andoxidizing gas is passed into the well whereby the combustion zoneadvances from the input well to each of the output Wells. For purposesof the example, only that portion of the formation will be consideredwith respect to the temperature distribution therein. However, it willbe understood that similar temperature distribution will occur withinsimilar segments of the formation 10.

Oxidizing gas is passed into the formation until the combustion zone hasadvanced forty percent of the distance along the line 21 between theinput well 15 and the output well 14, at which time the supply ofcombustion gas is discontinued. By means of thermocouples, thetemperature over that portion of the segment 20 represented by thevolume 22 bounded by the surface 23 in FIGURE 2 was determined to bell00 F. on the average.

With the supply of combustion gas discontinued and heat permitted totransfer from the portion of the formation behind the combustion zone tothe portion of the formation not yet traversed by the combustion zonewithout any assistance from injection of fluid, the heat distribution ata period of time one year following discontinuance of the supply ofcombustion gas is as indicated in FIGURE 3. At the surface 24 thetemperature is 250 E, at the surface 25 the temperature is 200 F, and atlocations between surfaces 24 and 25 the temperature varies between 200and 250 F. depending upon the distance between the surfaces. At thesurface the temperature is 175 -F., and at locations between thesurfaces 30 and 25, the temperature varies between 175 and 200 F.depending upon the distance between the surfaces.

Two years after the supply of oxidizing gas to the formation has beendiscontinued and transfer of heat is permitted to take place withoutassistance from injection of fluid, the temperature distribution withinthe formation is as indicated in FIGURE 4. Over surface 24, thetemperature is 250 F., over surface 25 the temperature is 200 F., andover surface 26 the temperature is 175 F,, and it will be noted that the17 5 F. surface has advanced a considerable distance toward the outputwell and the lower portion thereof has extended halfway to the top ofthe output well. As a result of the increase in temperature of thegreater portion of the formation from its original temperature of 100F., the hydrocarhon material within the formation is considerablyreduced in viscosity with resultant improvement in ease of production.Injection of fluid at this time into the input Well 15 assists indriving the hydrocarbon material within the formation to the output well14.

The invention may be employed in connection with the combustion processfor the recovery of petroleum oil from partially depleted subterraneanpetroleum oil reservoirs where the reservoir energy has decreased to thepoint that oil is no longer forced to the surface and particularly tothe point that the rate of recovery by pumping is so low as to beuneconomical. The invention may also be employed in connection with thecombustion process for the recovery of petroleum oil from a subterraneanreservoir wherein the oil has such high viscosity that efficientrecovery by water drive or other conventional means cannot be effected.Further, the invention is applicable in connection with the combustionprocess for the recovery of hydrocarbon materials from earth formationssuch as tar sands, for example, those existing in the Athabasca regionof Canada and elsewhere.

This application is a continuation-in-part of our copending applicationSerial No. 371,291, filed July 30, 1953.

Having thus described our invention, it will be understood that suchdescription has been given by way of illustration and example and not byway of limitation, reference for the latter purpose being had to theappended claims.

What is claimed is:

1. In a process for the recovery of hydrocarbon materials from an earthformation wherein an oxidizing gas is passed to said formation throughan input well leading thereto and forced through said formation to atleast one output well leading therefrom, combustion of a portion of thehydrocarbon materials is maintained within said formation, and acombustion zone is advanced through said formation as a result of thepassage of said oxidizing gas, the improvement comprising passing saidoxidizing gas to said formation through said input well in the directionof said output well until said combustion zone as a result of thepassage of said oxidizing gas to said formation has advanced more than40 percent but less than percent of the distance on a straight linebetween said input well and said output well, discontinuing passage ofgas through said input well to said formation for a period of time whichis appreciable and is at least as great as one month thereby permittingheat without assistance from injection of fluid in the direction of saidoutput well during said period of time to transfer from the portion ofsaid formation traversed by said combustion zone to the portion of saidformation in the direction of said output well not traversed by saidcombustion zone, and thereafter, to assist in production of hydrocarbonmaterials from said output well, passing fluid through said formation tosaid output well under conditions that combustion of said hydrocarbonmaterials in said formation is absent.

2. In a process for the recovery of hydrocarbon materials from an earthformation wherein an oxidizing gas is passed to said formation throughan input well leading thereto and forced through said formation to atleast one output well leading therefrom, combustion of a portion of thehydrocarbon materials is maintained within said formation, and acombustion zone is advanced through said formation as a result of thepassage of said oxidizing gas, the improvement comprising passing saidoxidizing gas to said formation through said input well in the directionof said output well until said combustion zone as a result of thepassage of said oxidizing gas to said formation has advanced more than40 percent but less than 90 percent of the distance on a straight linebetween said input well and said output well, discontinuing passage ofgas through said input well to said formation for a period of time whichis appreciable and is at least as great as one month thereby permittingheat without assistance from injection of fluid in the direction of saidoutput well during said period of time to transfer from the portion ofsaid formation traversed by said combustion zone to the portion of saidformation in the direction of said output well not traversed by saidoombustion zone until the temperature of said formation at said outputwell has risen measurably above its temperature existing at the timesaid passage of gas to said input well was discontinued, and thereafter,to assist in production of hydrocarbon materials from said output well,passing fluid through said formation to said output well underconditions that combustion of said hydrocarbon materials in saidformation is absent.

3. In a process for the recovery of hydrocarbon materials from an earthformation wherein an oxidizing gas is passed to said formation throughan input well leading thereto and forced through said formation to atleast one output well leading therefrom, combustion of a portion of thehydrocarbon materials is maintained Within said formation, and acombustion zone is advanced through said formation as a result of thepassage of said oxidizing gas, the improvement comprising passing saidoxidizing gas to said formation through said input well in the directionof said output well until said combustion zone as a result of thepassage of said oxidizing gas to said formation has advanced more than40 percent but less than 90 percent of the distance on a straight linebetween said input well and said output well, discontinuing passage ofgas through said input well to said formation for a period of time whichis appreciable and is at least as great as one month thereby permittingheat without assistance from injection of fluid in the direction of saidoutput well during said period of time to transfer from the portion ofsaid formation traversed by said combustion zone to the portion of saidformation in the direction of said output well, and thereafter, toassist in production .of hydrocarbon materials from said output well,passing a fluid which will not support combustion of said hydrocarbonmaterials in said formation through said formation to said output well.

4. In a process for the recovery of hydrocarbon materials from an earthformation wherein an oxidizing gas is passed to said formation throughan input well leading thereto and forced through said formation to atleast one output well leading therefrom, combustion of a portion of thehydrocarbon materials is maintained Within said formation, and acombustion zone is advanced through said formation as a result of thepassage of said oxidizing gas, the improvement comprising passing saidoxidizing gas to said formation through said input well in the directionof said output well until said combustion zone as a result of thepassage of said oxidizing gas to said formation has advanced more than40 percent but less than percent of the distance on a straight linebetween said input well and said output Well, discontinuing passage ofoxidizing gas through said input well to said formation for a period oftime which is appreciable and is at least as great as one month therebypermitting heat without assistance from injection :of fluid in thedirection of said output well during said period of time to transferfrom the portion of said formation traversed by said combustion zone tothe portion of said formation in the direction of said output well untilthe maximum temperature in the formation between the input well and theoutput well in the path of the flow of fluid through said formation isbelow the ignition temperature of said hydrocarbon materials in saidformation, and thereafter, to assist in production of hydrocarbonmaterials from said formation, passing fluid through said formation tosaid output well at a temperature below the ignition temperature of saidhydrocarbon materials in said formation.

5. In a process for the recovery of hydrocarbon materials from an earthformation wherein an oxidizing gas is passed to said formation throughan input well leading thereto and forced through said formation to atleast one output well leading therefrom, combustion of a portion of thehydrocarbon materials is maintained within said formation, and acombustion zone is advanced through said formation as a result of thepassage of said oxidizing gas, the improvement comprising passing saidoxidizing gas to said formation through said input well in the directionof said output well until said combustion zone as a result of thepassage of said oxidizing gas to said formation has advanced more than40 percent but less than 90 percent of the distance on a straight linebetween said input well and said output well, discontinuing passage ofgas through said input well to said formation for a period of time whichis appreciable and is at least as great as one month thereby permittingheat without assistance from injection of fluid in the direction of saidoutput well during said period of time to transfer from the portion ofsaid formation traversed by said combustion zone to the portion of saidformation in the direction of said output Well not traversed by saidcombustion zone until the temperature of said formation at said outputwell has risen at least 50 F. above its temperature existing at the timesaid passage of gas to said input well was discontinued, and thereafter,to assist in production of hydrocarbon materials from said output well,passing fluid through said formation to said output well underconditions that combustion of said hydrocarbon materials in saidformation is absent.

References Cited in the file of this patent UNITED STATES PATENTS2,584,606 Merriam et al. Feb. 5, 1952 2,642,943 Smith et al. June 23,1953 2,788,071 Pelzer Apr. 9, 1957 UNITED STATES PATENT OFFICECERTIFICATE, OF CORRECTION Patent No. 3,036,632 May 29, 1962 RobertLrKoch et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 5, line 41, before "well" insert input Signed and sealed this16th day of October 1962.

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER Commissioner of Patents Attesting Officer

1. IN A PROCESS FOR THE RECOVERY OF HYDROCARBON MATERIALS FROM AN EARTHFORMATION WHEREIN AN OXIDIZING GAS IS PASSED TO SAID FORMATION THROUGHAN INPUT WELL LEADING THERETO AND FORCED THROUGH SAID FORMATION TO ATLEAST ONE OUTPUT WELL LEADING THEREFROM, COMBUSTION OF A PORTION OF THEHYDROCARBON MATERIALS IS MAINTAINED WITHIN SAID FORMATION, AND ACOMBUSTION ZONE IS ADVANCED THROUGH SAID FORMATION AS A RESULT OF THEPASSAGE OF SAID OXIDIZING GAS, THE IMPROVEMENT COMPRISING PASSING OFSAID OXIDIZING GAS TO SAID FORMATION THROUGH SAID INPUT WELL IN THEDIRECTION OF SAID OUTPUT WELL UNTIL SAID COMBUSTION ZONE AS A RESULT OFTHE PASSAGE OF SAID OXIDIZING GAS TO SAID FORMATION HAS ADVANCED MORETHAN 40 PERCENT BUT LESS THAN 90 PERCENT OF THE DISTANCE ON A STRAIGHTLINE BETWEEN SAID INPUT WELL AND SAID OUTPUT WELL, DISCONTINUING PASSAGEOF GAS THROUGH SAID INPUT WELL TO SAID FORMATION FOR A PERIOD OF TIMEWHICH IS APPRECIABLE AND IS AT LEAST AS GREAT AS ONE MONTH THEREBYPERMITTING HEAT WITHOUT ASSISTANCE FROM INJECTION OF FLUID IN THEDIRECTION OF SAID OUTPUT WELL DURING SAID PERIOD OF TIME TO TRANSFERFROM THE PORTION OF SAID FORMATION TRAVERSED BY SAID COMBUSTION ZONE TOTHE PORTION OF SAID FORMATION IN THE DIRECTION OF SAID OUTPUT WELL NOTTRAVERSED BY SAID COMBUSTION ZONE, AND THEREAFTER, TO ASSIST INPRODUCTION OF HYDROCARBON MATERIALS FROM SAID OUTPUT WELL, PASSING FLUIDTHROUGH SAID FORMATION TO SAID OUTPUT WELL UNDER CONDITIONS THATCOMBUSTION OF SAID HYDROCARBON MATERIALS IN SAID FORMATION IS ABSENT.